Movement of a body in a straight line while in a rotating frame of reference produces a perpendicular force known as a Coriolis force. This phenomenon can be utilized in inertial sensing by obtaining a signal that is a measure of the Coriolis acceleration. In applications such as inertial navigation, for instance, linear accelerometers can be used to sense the Coriolis acceleration. Conventional high performance inertia measurement units have typically been expensive, complex clusters of three gyroscopes and three accelerometers. More recently, the advent of low cost, high volume micromachined accelerometers has made it cost effective to use only accelerometers to perform the entire inertial sensing function. These accelerometers are preferably vibrated or dithered in the rotating frame of reference, which produces the sensor-induced Coriolis force, and thereby produces the sensitivity of the device to rotation. If the dither amplitude and frequency are held constant, the Coriolis acceleration is proportional to rotation rate, which can therefore be directly measured by the accelerometers.
The above-described principle can be used to measure the angular rate and linear acceleration with respect to three orthogonal prime axes X, Y and Z of a body by providing an accelerometer in association with each of the axes. Thus, periodic or dithered movement of an accelerometer along the Y axis of the body, with the sensitive axis of the accelerometer aligned with the Z axis, results in the accelerometer experiencing a Coriolis acceleration directed along the Z axis as the body rotates about the X axis. This acceleration or force is proportional to a change in velocity of the body along the Y axis and its angular rate of rotation about the X axis. An output signal from the accelerometer thus includes a force signal representing the linear acceleration of the body along the Z axis, and a periodic component or rotational signal representing the Coriolis acceleration resulting from rotation of the body about the X axis. Normally, the amplitude of the Coriolis force signal would be very small in relation to the linear acceleration signal, but dithering the accelerometers at a specific frequency helps to detect the Coriolis force signal.
When using dithered accelerometers to detect Coriolis force for thereby measuring the rate of rotation of a body, it is essential to separate the dither frequency signal from other sources of acceleration. With a linear accelerometer, a large body acceleration can be summed with a small Coriolis dither signal. If accelerometers are paired in opposed relationship to one another, the linear acceleration can be separated from the angular rotation induced acceleration by sum and difference techniques in a signal processor. In addition, such pairing of accelerometers enables the signal processing to eliminate spurious signals, since each accelerometer in the pair will produce an equal but opposite signal for any extraneous forces on the accelerometers, such as vehicle vibration and the like.
The foregoing principles have been incorporated in several prior art inertial sensing devices, including those described in commonly owned U.S. Pat. Nos. 4,510,802, 4,590,801 and 4,821,572. Reference can be made to those patents for a detailed explanation of the theory of operation using dithered accelerometers to measure angular rate and linear acceleration. These patents variously disclose methods and apparatus for mounting and dithering accelerometers and for the processing of the output signals from the accelerometers to obtain angular rate and linear acceleration measurements of a moving body on which the accelerometers are mounted.
U.S. Pat. No. 4,821,572, for example, discloses an arrangement in which pairs of opposed accelerometers are mounted in a triaxial configuration for detecting and measuring linear acceleration and angular rate of motion about a plurality of prime axes. In this arrangement, a matched pair of accelerometers is associated with each of the prime axes, with one accelerometer of each pair mounted on a first rotating frame member and the other accelerometer of the pair mounted on a second, counter-rotating frame member. The acceleration sensitive axes of the two accelerometers comprising a pair are perpendicular to the direction of dither movement and parallel with a respective prime axis. They are also parallel to one another but point in opposite directions. Accordingly, each accelerometer produces an output signal that is a function of the Coriolis acceleration. Signal processing means connected with the accelerometers are operative to determine the angular rate of rotation and linear acceleration with respect to the associated prime axis. Since the accelerometers of each pair are disposed in back-to-back or opposed relationship to one another, equal but opposite signals are produced by the two accelerometers. This matching of pairs of opposed, dithered accelerometers, in conjunction with the unique signal processing, enables common mode error signals to be rejected while at the same time increasing the sensitivity of the sensor to the motions being measured, as more fully explained in the referenced patents.
The preferred arrangement of the accelerometers in U.S. Pat. No. 4,821,572 is depicted in FIGS. 12 and 13. As shown, the housing on which the accelerometers are mounted has a dither axis 40 about which the frames and their associated accelerometers are oscillated, and the sensitive axes of the accelerometers are disposed at an acute angle of 35.26.degree. with respect to this axis. The centers of percussion of the accelerometers in this structure are spaced apart from one another by a distance "d.sub.1 ", and when the accelerometers are dithered about the axis 40, their output signal will include an error component proportional to this distance and to the angular acceleration of the accelerometers about an axis perpendicular to both the rate sensitive axis and the acceleration sensitive axis. This distance could be reduced by mounting the accelerometers closer to each other.
But, moving the accelerometers closer together is prevented by the multiple electromagnets 42 and associated hardware which are used in the structure of U.S. Pat. No. 4,821,572 to provide the magnetic force for obtaining the dither motion, and to establish the flux field that cooperates with pick-off coils 66 to sense the dither motion. A variety of mounting hardware and separately attached pole pieces, with concomitant assembly steps are required in this structure.
There is, therefore, need for an angular rate and linear acceleration sensor that overcomes the above deficiencies in the prior art by providing a structure that is simple and compact, economical in construction, and which provides increased sensitivity while at the same time reducing error signals due to misalignment, cross-axis motion and the like.